GB2118483A

GB2118483A - Insulating material for the windings of a coil of metallic foil

Info

Publication number: GB2118483A
Application number: GB08309009A
Authority: GB
Inventors: Friedrich Schindelmeiser
Original assignee: AEG Isolier und Kunststoff GmbH
Current assignee: AEG Isolier und Kunststoff GmbH
Priority date: 1982-04-01
Filing date: 1983-03-31
Publication date: 1983-11-02
Also published as: GB2118483B; GB8309009D0; DE3212060A1

Abstract

An insulating material usable as an insulation for windings of aluminium foil (14) of a coil comprises a compound material comprising a porous carrier material (10) not shrinking in heat, mica paper (11) and a curable reaction resin, the material being so deformable in thickness by the pressure stresses arising during winding of the coil under tension that an intimate contact of the material with the aluminium foil (14) occurs at both sides and at all places. The quantity of resin is chosen so that a secure bonding with the foil is achieved at both sides and that the insulating material itself has a sufficiently high mechanical strength and a higher linear thermal coefficient of expansion perpendicularly to the layer direction than the aluminium foil. <IMAGE>

Description

SPECIFICATION Insulating material for the windings of a coil of metallic foil The present invention relates to an insulating material for insulating the windings of a coil of metallic foil, especially aluminium foil, such a coil being usable in, for example, a dry transformer of temperature classes Fand H.

For winding insulation in low voltage windings of aluminium foil, it has been usual to use glass fabrics pre-impregnated with a hardenable binding means on the basis of epoxide resin, esterimide resin and modified esterimide resin. For windings with higher winding voltages, paperlike webs of aromatic polyamide lacquered on both sides with a hardenable binding means on the basis of epoxide resin and polyesterimide resin are used for insulation. The sheet insulating material thus produced is tightly wound together with aluminium foil on a winding core to form, in general, a cylindrical coil. The curing of the binding means is then effected through appropriate oven heat treatment of the coil. The winding core is removed after cooling. In order to ensure a sufficiently high flash-over voltage strength between the windings at their end faces, the insulating material is arranged to protrude.The end faces of the windings are as a rule cast in an appropriate casting resin mass. As a result, the protruding winding insulation is mechanically protected. It is also usual to cast the entire coil in a resin mass.

For the operational reliability of these windings, the following points are important for the winding insulation: a) The insulating material must be free of fault zones of any kind so as to withstand the winding voltage loadings and the surge voltage loadings arising during the operation of, for example, a transformer incorporating the coil.

b) The insulating material should firmly stick to the aluminium foil on both sides after curing, as a sufficient mechanical strength, especially in the case of short-circuit loadings, is ensured only by this means.

Moreover, the strength of the bonding should be maintained up to the maximum operating temperature.

c) For satisfactory conducting away of heat, the thermal conductivity of the insulating material should be good and the bonding should be fault-free at both sides.

In the production of the pre-impregnated glass fabric, glass fabric webs are as a rule impregnated with a solution of resin and hardner. The solvents are then expelled as far as possible and the binding means is made to react slightly through subsequent heat treatment in a vertical shaft. The inclusion of small air bubbles in the fabric cannot be entirely avoided in these pre-impregnated glass fabrics. This gives rise to electrical fault locations, which are generally known as "windows". Insulating materials of that kind are therefore usable only for relatively low electrical loadings. In the case of the pre-impregnated glass fabrics, it is also disadvantageous that the surfaces are never quite uniform and smooth and it is not possible to avoid protrusion of individual fibres from the glass fabric.At the locations of such protrusions, thickened zones in the form of small knots form at the surfaces. Moreover, pre-impregnation of that kind is as a rule relatively hard. In the manufacture of windings from aluminium foil and preimpregnated glass fabric, due to the irregular surfaces and the thickenings in the form of small knots, a smooth contact of the aluminium foil with the fabric is never achieved during the winding. The thickenings in that case act as spacers. Examination of such windings with pre-impregnated glass fabrics has shown that the insulation usually adheres only at one-side to the aluminium foil. The reasons for this are the above-mentioned non-uniform surface and above all the thickenings in the form of small knots. Due to this, optimum mechanical strength is not achieved in windings of that kind.Moreover, because of the insufficient contact between the winding insulation and the aluminium foil, the heat removal capability is reduced. Paper-like webs of aromatic polyamide are known by the Trade Marks ARAMID Paper and NOMEX. The type Nomex 410 is the most usual. As is stated for this by the manufacturer, the firm of Du Pont, this material absorbs moisture from the air and thereby changes in length.

Example forNomex 410 with a thickness of 0.25 millimetres relative air Increase in Expansion (%) humidity weight LR QR (Length) (Breadth) % % Oven-dry 0 0 0 50 3.5 0.4 0.5 65 5.1 0.6 0.9 95 8.4 1.1 1.8 After lacquering both sides of the aromatic polyamide webs with the previously mentioned reaction resins, the absorption capacity for moisture remains almost the same. If materials of that kind are employed as a winding insulation for windings of aluminium foil, then when the resin is cured the moisture in the insulation is largely expelled. As a result, shrinkage takes place in correspondence with the abovementioned expansion values. The usual consequence is strong distortion within the windings, whereby a very high reject rate is met with.Moreover, due to the resulting shrinkage, the windings are so strongly pressed against the winding core disposed in the winding during the curing that removal of the winding without damage is almost impossible. Such difficulties are particularly pronounced in insulating materials of that kind for the heat class H, since the curing termperatures in this case are correspondingly high. Further difficulties with windings insulated in that manner occur in the curing of the casting resins utilized for the embedding of the end faces. Large bubbles arise in the grouting due to the small quantities of moisture and gas still escaping from the aromatic polyamide.

Insulations for windings of aluminium foil are at present used in various widths between about 400 millimetres and about 1500 millimetres. In future, even wider materials, up to about 1900 millimetres, may be required. However, the available standard widths of the most common aromatic polyamide paper are 610,914 and 1500 millimetres, which means that there is a correspondingly large wastage in the case of insulation materials having intermediate width, which entails additional costs. At present, it is not possible to produce widths above 1500 millimetres.

No great difficulties exist with glass fabrics in respect of maximum widths above 1500 millimetres. In respect of waste, however, the problems are similar two those connected with aromatic polyamide papers.

There is accordingly a need for an insulation material which is usable as winding insulation for windings of, for example, aluminium foil and which is free of the disadvantages of the known materials, i.e. the strength of the bonding of the insulating material to the foil should be secured on both sides up to the maximum operating temperature, the thermal conducitivity of the insulating material should be improved and fault zones in the insulation should be avoided as far as possible.

According to the present invention there is provided an insulating material for insulating the windings of a coil of metallic foil, the insulating material comprising a composite material, which comprises a layer of mica paper, a layer of a porous carrier material resistant to heat shrinkage, and a curable reaction resin, and which is so deformable in thickness under pressure exerted during winding under tension of metallic foil together with the composite material to form the coil that the composite material is disposed in intimate overall contact at both sides with the foil, the resin being pesent in such an amount as to ensure bonding with the foil at both sides and the composition of the composite material being such that it has a pedetermined mechanical strength and a higher linear coefficient of thermal expansion perpendicular to the winding direction than the foil.

The invention also embraces a coil comprising a foil of metallic material, for example aluminium, wound under tension and an insulating material as described in the preceding paragraph disposed between the windings of the coil in intimate overall contact and bonded at both sides with the foil.

Embodiments of the present invention will now be more particularly described by way of example and with reference to the accompanying drawings, in which: Figure 1 is a sectional view of a first sheet insulating material embodying the invention; Figure 2 is a sectional view of a second sheet insulating material embodying the invention; and Figure 3 is a sectional view of part of a coil winding incorporating the insulating material of Figure 2.

Referring now to the drawings, there is shown in Figure 1 a mica foil insulating material for insulating the windings of an aluminium foil coil usuablefora dry transformer, the insulating material comprising a porous carrier material 10, for example glass fibre fabric, glass fibre quilt etc., not shrinkable by heat, a mica paper 11, and a curable reaction resin, preferably on epoxide resin base, which impregnates the carrier material and the mica foil. The properties of the insulating material are adjusted so that it remains ductile or somewhat deformable in the thickness under pressure at room temperature. This is achieved through appropriate setting of the curable reaction resin, thus the binding means for the carrier material and mica paper.

In the case where the coil is to be used for a transformer in heat class F, the reaction resin may comprise an epoxide novolak resin. For example, the reaction resin may consist of an epoxidised novolak, which is firm at room temperature and has an epoxide equivalent of 175 to 180, in compound with a liquid bisphenol epoxide resin or a liquid cycloaliphatic epoxide resin on carboxylate base for setting the required deformability of the insulating material. If the coil is to be used in a transformer in heat class H, the reaction resin may comprise a cycloaliphatic epoxide resin.For example, the reaction resin may then consist of a cycloaliphatic epoxide resin, which is firm at room temperature, is precondensed with an anhydride hardener in deficiency and has an epoxide equivalent of about 215, in compound with a liquid cycloaliphatic epoxide resin on cargboxylate base again for setting the required deformability. In both cases, a BF3 adduct is used as a reaction catalyst.

For improved utilization of materials or reduction of waste, as shown in Figure 2 the carrier material 10 and mica paper 11 can be in the form of strip lengths 12, which abut one against the other and which are covered at their butt joints by an electrically and thermally high grade thin insulating film 13, for example polyimide film. This film can be applied in the form of a wider tape and bonded to the strip lengths 12 by the binding means, i.e. resin, present in the layers of carrier material and mica paper. The film avoids creation of electrical fault zones.

The afore-mentioned requirements (a) to (c) may be completely fulfilled by an insulating material of either of the described embodiments.

Insulating material embodying the invention can be produced in roll form in any desired thicknesses from 0.09 millimetres to 0.4 millimetres and in widths up to about 1.9 metres or more.

In the case of an insulating material intended for use in connection with heat class F, after curing for about 16 hours at 150"C a tan value of about 0.08 at 155"C is achieved. This also indicates that the mechanical strength value does not fall off to any extent up to 1 550C. Thus, coil windings insulated by such insulating material have a good short-circuit strength even at 1550C.

In the case of an insulating material intended for use in connection with heat class H, after curing for one hour at 1700C a tan 5 value of about 0.06 at 1 550C is achieved. After further tempering for abot 100 hours at 150"C, a tan 6 value of about 0.08 at 1 800C is obtained, which means that after a relatively short operating time, a mechanical strength value up to 180"C is virtually maintained and windings insulated with this insulating material have good short-circuit strength up to 180"C.

In the case of the insulating material shown in Figure 2, the covering film 13 may be, for example, a polyimide film tape with a width of 50 millimetres and a thickness of 0.025 millimetres. This film has a breakdown voltage of about 5.6 kilovolts. The butt joint is accordingly completely electrically insulated.

During production of the windings with aluminium foil 14, the thin film 13 is pressed into the insulating material layers, as shown in Figure 3, so that the butt joints covered by the film do not stand out. The binding resin in the insulating material becomes so highly liquid during the curing that this permeates between covering film 13 and adjacent aluminium foil 14 so that a secure bonding is also achieved at these locations.

The following are examples of constructions of the insulating material at different thicknesses.

Normal thickness in millimetres 0.09 0.15 0.18 0.21 0.29 0.40 Mica paper (approximate grams persquare metre): 50 120 120 160 250 367 Glass fabric (approximate grams per square metre): 26 26 35 35 43 43 Binding means (resin) (approximate grams per square metre): 49 91 97 122 183 255 Total weight (approximate grams per square metre): 123 237 252 317 476 667 In the use of such an insulating material with a thickness of 0.18 millimetres, an alternating voltage strength of 2 to 3 kilovolts and a surge voltage strength of 4.5 to 6.0 kilovolts is achieved with a single layer of the material between aluminium foil.

In the constructions described above, a saturated and completely secure bonding with the aluminium foil is achieved at both sides of the insulating material. Materials of that kind have a linear thermal coefficient of expansion of about 30.1 0-6/0C. As a result, an additional compression of the insulating material occurs at the curing temperatures of the binding resin, which is of advantage for the securing bonding with the aluminium foil. After the curing of the resin, mechanical stresses can be resiliently absorbed by the windings. Detaching of the insulation from the aluminium foil does not occur even after extended operating periods. Access of air to the winding insulation is practically impossible in view of the generally usual embedding of the coil and faces in casting material as well as through the aluminium foil and the securing bonding between the foil and the winding insulation. Under these conditions, a continuous temperature strength of the winding insulation is, according to experience, greater by about 10 C than when access by air is possible.

With such winding insulations, the thermal conducitivity may be 0.30 to 0.32 watts per metre per "C.

The carrier material may also comprise fibre bundles, arranged beside each other or at certain spacings in longitudinal direction, of glass filament.

Claims

1. An insulating material for insulating the windings of a coil of metallic foil, the insulating material comprising a composite material, which comprises a layer of mica paper, a layer of a porous carrier material resistant to heat shrinkage, and a curable reaction resin, and which is so deformable in thickness under pressure exerted during winding under tension of metallic foil together with the composite material to form the coil that the composite material is disposed in intimate overall contact at both sides with the foil, the resin being present in such an amount as to ensure bonding with the foil at both sides and the composition ofthe composite material being such that it has a predetermined mechanical strength and a higher linear coefficient of thermal expansion perpendicular to the winding direction than the foil.

2. An insulating material as claimed in claim 1, wherein the carrier material comprises a fibreglass fabric.

3. An insulating material as claimed in claim 1, wherein the carrier material comprises a glass fibre quilt.

4. An insulating material as claimed in either claim 2 or claim 3, wherein the resin comprises an epoxidised novolak, which is firm at room temperature and has an epoxide equivalent of 175 to 180, in compound with one of a liquid bisphenol epoxode resin and a liquid cycloaliphatic epoxide resin on a carboxylate base.

5. An insulating material as claimed in either claim 2 or claim 3, wherein the resin comprises a cycloaliphatic epoxide resin, which is firm at room temperature, has an epoxide equivalent of substantially 215 and is precondensed with an anhydride hardener in deficiency, in compound with a liquid cycloaliphatic epoxide resin on a carboxylate base.

6. An insulating material as claimed in either claim 4 or claim 5, wherein the resin further comprises a boron trifluoride reaction catalyst.

7. An insulating material as claimed in any one of the preceding claims, wherein the mica paper and the carrier material are in the form of strip lengths disposed in end-to-end relationship and abutting at adjacent ends, and the composite material further comprises an electrically and thermally insulating film covering the abutting ends and connected to the strip lengths by the resin.

8. An insulating material as claimed in claim 7, wherein the film is polyimide film.

9. An insulating material as claimed in any one of the preceding claims, wherein the carrier material comprises bundles of fibreglass filaments disposed one beside the other.

10. An insulating material as claimed in any one of claims 1 to 8, wherein the carrier material comprises bundles of fibreglass filaments disposed at spacings in the winding direction.

11. An insulating material substantially as hereinbefore described with reference to Figure 1 of the accompanying drawings.

12. An insulating material substantially as hereinbefore described with reference to Figure 2 and 3 of the accompanying drawings.

13. A coil comprising a foil of metallic material wound under tension and an insulating material as claimed in any one of the preceding claims disposed between the windings of the coil in intimate overall contact and bonded at both sides with the foil.

14. A coil as claimed in claim 13, wherein the metallic foil is aluminium foil.

15. A coil as claimed in claim 13, and substantially as hereinbefore described.